The mass of the two test satellites being launched this month is 400 kg, but I don't believe that includes the antennae and the structure to hold and release them. Is total mass the only constraint or would the volume further constrain the number of satellites?

$\begingroup$Volume is the issue, I have seen estimates as low as 20 and as high as 40. The volume is the big issue, but have not seen anything official that is worthy of answering.$\endgroup$
– geoffcFeb 20 '18 at 14:39

3 Answers
3

Something I did not take into account in these calculations is the likely reduction in weight of the satellite during the mass production. For example, the Orbcomm OG2 fleet weighs in at 172 kg per satellite. This is a likely goal for SpaceX's satellites (and this would allow twice as much satellites per launch).

Also, the satellites are already fitted with antennas. Phased array antennas can be really tiny!

Falcon 9

Payload Mass

As per the Falcon 9 User Guide, the payload to LEO when flying fully expendable is 22 800kg. However, it's unlikely SpaceX will orbit their satellite fleet (all 12 000 of them, from this link) flying expendable. The Falcon 9 Wikipedia page cites a 9 600kg payload capability in the reusable configuration.

This would amount to a staggering 24 satellites per launch.

We know SpaceX has already launched thirty new satellites of the IridiumNEXT constellations in three launches. Their injection orbit is not far off the one of SpaceX Starlink, and the satellites weighs in at about 860kg (as per this article).

This is consistent with our findings too.

However, as you mentioned, all of this did not account for the payload attach fitting (the thing to hold and release the satellites).

If we use again the Iridium flights as reference, the PAF configured with 10 satellites was about 1 000kg max. This would drive the available payload mass down to 21.5 satellites.

I will round that down to 20 satellites, arranged in 5 rows of 4, with an alternate pattern (i.e. first row North/South/East/West, next row NE/NW/SE/SW, etc...) since they will want to have them detach from the PAF two by two in a coordinated fashion.

Moreover in this configuration, the PAF can weigh up to 1 600kg, which should be plenty enough for 20 satellites smaller than Iridium's.

Physical

However, we also have to check those would fit in the fairings. The Falcon 9 User Guide also gives us the internal dimensions of the fairing. Fitting 5 rows of 4 satellites should not be a problem here.

View from top:

Also, this would amount to a total of 600 launches on the Falcon 9.

Some people suggest that volumetrically, 40 could fit as follows:

Falcon Heavy

Payload Mass

The Wikipedia page for Falcon Heavy, and available info on SpaceX's own website, does not reference a payload mass when flying fully reusable. I will assume it's the same ratio as with the Falcon 9. When flying reusable vs fully expendable, payload mass is reduced by 2.375 (22 800 / 9 600).

The maximum payload mass is 63 800 kg when flying expendable, so this would gives us a payload mass of roughly 26 800kg when flying reusable.

This would allow us to send 64 satellites in orbit (exact number is 67, but I want to keep the *4 or *8 symmetry.).

64 satellites would gives us a margin of 1 600kg for the PAF. Seems a bit light to me for 64 satellites.

Physical

Regarding the physical space, we can assume the Falcon Heavy fairings are about the same size as the Falcon 9 ones (same core diameter).

If we fit 8 satellites per row, that would gives us 8 rows of satellites. It's starting to get a bit cramped, especially near the top. However, with some special arrangement, I believe this could still be done.

This configuration and the launches on Falcon Heavy would amount to 188 launches.

BFR

Payload Mass

This is going to be a bit easier but less precise. The Wikipedia page for BFR gives a maximum payload mass of 150 000kg when flying reusable. (No, that is not a typo. It is 150 metric tons to LEO.)

This would allow us to send 368 satellites in orbit (exact number is 375, but again here, I want to keep the *4 or *8 symmetry).

368 satellites would gives us a margin of 2 800kg for the PAF. Definitely, that is not enough for that number of sats.

I reckon the number of satellites per launch on the BFR will be in the ballpark of 304 satellites (this would make the PAF weigh up to 100kg/satellite, same as the IridiumNEXT launches).

Physical

Core diameter is 9m. The rest of the available payload space is not known.

However, fitting 304 satellites in 19 rows of 16 satellites should be possible I believe.

$\begingroup$Glad to help. People forget that the purpose of Stack Exchange is get good answers. I am a points piggy, but still glad to add interesting info to other peoples questions even when I get no points. :)$\endgroup$
– geoffcFeb 21 '18 at 14:58

1

$\begingroup$@geoffc Elon Musk has tweeted that soon the Fairing 2 will be used, which has larger diameter. Also, in the future, the fairing might be made "much longer" twitter.com/elonmusk/status/963095860060934144 How much would the diameter have to be increased to add another row of satellites to the above scenario?$\endgroup$
– Everyday AstronautMar 5 '18 at 16:54

$\begingroup$@derwodamaso Aye theres the rub! Fairing 2 has a minorly larger diameter. My understanding is a matter of inches, not meters. Longer would help more in the short term, but there is a limit for how long the vehicle can be, and it has been suggested it is at the max of possible length already.$\endgroup$
– geoffcMar 5 '18 at 17:13

$\begingroup$@geoffc You seem to be impressingly well-informed. Is that publicly accessible information? Also, if Fairing 2 is wider really just by inches, that teases a follow-up question...$\endgroup$
– Everyday AstronautMar 5 '18 at 22:04

$\begingroup$I follow Reddit's r/spacex. I follow the Nasaspaceflight.com forums, and keep pictures I see and try to remember details. All public info as far as I know. I do not knonw anyone at SpaceX. Alas. Wish I did!!!$\endgroup$
– geoffcMar 5 '18 at 22:41

I agree with the lift capacity of BFR at 304 Starlink satellites, by weight and probably dimensionally. But there is the problem of each orbital plane on the first Starlink Phase I only has 34 satellites in it. See the section on "How many launches...". But changing from one orbital plane to another costs a substantial amount of fuel, it is unlikely that BFR can change to 9 different orbital planes in one flight, though it is also possible that each satellite will have enough of a thruster to make the change for itself over the course of a few days.